No, and that's final! (5)
I believe the answer is:
never
'no' is the definition.
Both the answer and definition are adverbs. Maybe you can see a link between them that I don't see?
'and that's final' is the wordplay.
'and' becomes 'n' (common abbreviation for 'and').
'that's final' becomes 'ever' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'n'+'ever'='NEVER'
Can you help me to learn more?
(Other definitions for never that I've seen before include "Do not give up!" , "Emphatic negative" , "Under no circumstances" , "' - - Say - - Again': Bond film" , "Certainly not!" .)